Indus-1, Synchrotron Radiation Source (SRS) facility operating at 450 MeV, is operational at RRCAT, Indore. It emits radiation in VUV range and is being used for research in Physics and Chemistry. The SRS complex consists of 20 MeV injector microtron, 700 MeV booster synchrotron and 450 MeV storage ring. In all these machines the energy to the accelerating particle i.e. electrons, is imparted by the RF voltages developed across RF cavities. This article describes the microwave system for the microtron, and the RF systems for the booster synchrotron and storage ring INDUS-1. The RF System for the ion clearing electrodes and Radio Frequency Knock Out (RFKO) systems are also described. 

INTRODUCTION

The role of RF system, in synchrotron radiation source (SRS), is to compensate for SR losses suffered by circulating electrons through  the bending magnets and insertion devices to keep the beam in equilibrium orbit.  The RF system also provides energy for accelerating the beam from injection energy to final energy. Booster synchrotron and Indus-1 are relatively small rings with perimeters of 28.449 m and 18.996 m respectively. The operating frequency of both the rings is chosen as 31.613 MHz mainly to avoid ion trapping problems. The booster synchrotron is operating at harmonic number of three and storage ring is operating with harmonic number of two. Owing to same RF frequency, the cavities in both the rings are similar. The main parameters of both the rings are listed at Table-1.

The choice of relatively lower RF frequency dictated the use of heavily capacitively loaded RF cavities for both the rings. The synchrotron cavity is made out of Aluminium and that in the storage ring is made out of SS and plated internally with copper. In synchrotron as well as storage ring, the RF system consists of a synthesized signal generator followed by a chain of amplifiers, the low level control circuits for frequency, phase and amplitude control of the RF field across the cavities. The main power amplifier is built using indigenous BEL make power tetrode tubes.

Pillbox type of cavity operating at 2856 MHz is employed in the microtron to accelerate the electrons to 20 MeV energy. The cavity generates nearly 1000 kV accelerating voltage. 5MW microwave source is developed employing S-Band klystron tube, to power this cavity.

Table-1: Parameters of booster synchrotron and INDUS-1 storage ring. Figures in brackets correspond to the operation of the synchrotron at 700 MeV

RF SYSTEM

The RF system for the storage ring consists of a synthesized signal generator, the low level RF system, the high power amplifier along its power supply and the RF cavity. Figure 1 shows the block diagram of booster and storage ring RF system

The low level RF System

Low level RF control system consists of synthesized signal source,0-360^o phase shifter,  feed back loops for amplitude, phase and frequency control, Coaxial RF switch to put RF on & off and Limiter. Synthesized signal generator giving outputs at 31.613 MHz is developed which will also be used to get synchronized RF drive signal for Indus-2. Amplitude and phase control feedback loops are incorporated to maintain the amplitude and phase of the cavity gap voltage within 1% and 1^o respectively for proper operation of the machine. The phase loop compares   the phase of the RF signal at the feed of RF cavity with the reference signal phase. The error signal obtained is used to drive the variable phase shifter which corrects the phase changes in the amplifier chain. The electronic fast phase shifter is built using BEL make cascode / differential amplifiers BMC 3028 A and varacter diodes. To get the phase information at different operating power level limiter circuits have been used. These are designed to give constant output of +13dBm at with input ranging from -18 dBm to +13 dBm

 The amplitude control loop measures the accelerating field in the cavity sampled through small coupling loop. This sampled signal is compared with a reference (amplitude set) signal. The correction is made by driving the RF attenuator with the error signal, which in turn controls drive to solid state amplifier.

The tuning loop together with amplitude control loop compensates for the beam loading effect and temperature change. The tuning loop maintains the resonance frequency by comparing the phases between cavity voltage and cavity input signal from transmission lines feeding to the cavity. The phase detector gives equivalent error signal proportional to phase difference between the two signals. Resonance control is achieved by driving the plunger tuners with the help of stepper motors. Total dynamic range provided is 40 KHz.

During operation of both the RF systems manual access is not possible hence both R.F. amplifier systems are made to operate in local mode and remote mode from control room. All the parameters like cavity gap voltage, forward and reflected powers at cavity, tuner position, cavity tuning error etc. are monitored and  interlocks are provided for safe operation of the machine.

The RF Amplifier System

The operating frequency Booster Synchrotron & Indus1 storage ring is 31.613 MHz. Similar RF power amplifier scheme is employed for both the machines. The RF system consists of synthesized source, low level RF control system, 150 watts solid state driver amplifier, tetrode tube based Power amplifier, 7kV/2A regulated power supply along with many auxiliary supplies, 15/8 “ coaxial transmission line with circulator and  RF cavities. The storage ring is equipped with Radio frequency Knock Out (RFKO) amplifier systems & Ion clearing electrodes RF System operating in the frequency range of 1-10 MHz.

Fig. 2 The RF amplifier system showing the solid state amplifier, circulator, the power amplifier and the transmission lines.

The RF system ( Fig.2)  is built using indigenous components. In this system, the low level signal of 10 mW from frequency synthesizer is amplified to 150 watts by solid-state amplifier. The basic building block of solid-state amplifier is 80 watts amplifier module built using RF power transistor type S-100-28. To get good linearity, the solid state amplifier is designed to operate in class AB mode. The total power of 150 watts is obtained by combining 2 modules of 80 watts using hybrid power combiners. The low & medium power amplifier modules of 1 W & 10W are built using BEL make RF transistors 2 N 3866, 2 N 5070. These modules operate in class A mode. The twisted pair transmission line type transformers (BALUNS) are used to get wide bandwidth. These transformers are made using soft Ni-Zn ferrite core. The reflectometer with VSWR protection system is developed and incorporated in the system to monitor forward and reflected output power of solid-state driver amplifier. The circulator is incorporated at the output of SSA to protect it from reflected power from the power amplifier.

The final power amplifier is built using BEL make ceramic power tetrode tube type 4 CX15000. It operates in class B grid driven configuration.  Impedance matching networks with pi-configuration are used to match the input and output impedances of tube to standard 50 ohms.  Output matching network uses a high power choke and high voltage RF vacuum variable capacitors. The complete power amplifier is assembled in a copper shielded box (fig-3) to have proper RF ground and shielding.

The plate of power amplifier is supplied with a 7 kV/ 2A series regulated power supply(Fig-4). This power supply power employs water cooled triode tubes type 6000 WC giving better than 0.1% regulation. The series regulated power supply enables very fast switch off of plate bias of the tetrode tube.  1 kV / 100mA and –250V/300mA regulated power supplies feeds the screen grid and control grid of 4 CX 15000 tube. The output power from the power amplifier is transmitted to the cavity through 1⁵/₈“ coaxial line. High power Y-junction circulator is used to protect the expensive tetrode from reflected power from cavity because of mismatch. Co-axial directional couplers are developed and incorporated in the line and used for measuring forward and reflected power to and from the RF cavity.

The RF cavities

Design

Design parameters of rf cavities are listed in Table I The RF cavity is designed with the help of SUPERFISH[1]. The accelerating gap being only 20 mm, the transit time factor is @1 for v=c electrons at energies ³ 5 MeV. The frequency sensitivities of various cavity surfaces as calculated by SUPERFISH are as follows: cavity radius -0.0112 MHz/mm; cavity length -0.0305 MHz/mm; drift tube outer radius -0.06 MHz/mm; capacitive loading disk back surface -0.0216 MHz/mm and the accelerating gap 0.635 MHz/mm. Unlike the normal cavities the field H_j is maximum at the outer surface of the drift tube and not at the cavity radius. Therefore the frequency sensitivity of the cavity radius is much smaller as compared to that of the drift tube outer surface. Therefore we could squeeze the cavity diameter further to make it more compact. Figure 5 shows the geometrical details of INDUS-1 and booster cavity.

Provision for HOM damping

The study of field distributions of the Higher Order Modes  reveals that almost all of them have strong radial electric field on the cylindrical boundary. Therefore it was decided to use damping probes or antennae which will couple only to the radial electric field as the loop couplers are bound to couple to the fundamental mode which is undesirable. Based on this, there are in all five ports provided at these points on the cylindrical wall of the cavity for mounting the damping antennae.

               

4 RADIO frequency knock-out system

 (RFKQ) system is employed to measure betatron tunes in Booster & Indus-1 ring, In Booster Synchrotron pairs of open plate electrodes are installed as beam shaker. These electrodes require RF voltage of 0 -1000 volts at frequency range of 1 to 5 MHz. A wide band 200 watts RF amplifier has been used. The voltage level of 1000 volts is obtained by transmission line type voltage/ impedance transformers.  RFKO unit of Indus-l storage ring uses pairs of striplines installed as beam shakers electrodes.  Each strip line is excited

Fig 6. RF Knock out System for INDUS-1

with 100 volts RF signal with frequency variable from 1 to 10 MHz. The other strip line is excited with same signal with phase difference of 180^o. Indus-l RFKO amplifier system consists of two units of 300 watts wideband RF amplifier modules driven at 180° phase difference . 300 watts of RF amplifier unit is designed with BEL make RF power transistor S175-28 operating in class AB push pull mode. Two such units are driven by 180° power splitter.

5 ION clearing electrode RF system

 Circulating electron beam ionizes the residual gas molecules in the vacuum chamber of Indus-1. These positively charged particles remain trapped near the design orbit due to the attractive potential of the electron beam and may give rise to instability. To keep the ion concentration below a certain value, 8 numbers of Ion clearing electrodes are installed at various locations in the vacuum chamber. These electrodes are biased with 1 KV DC superimposed with   200 volts peak to peak RF voltage. The frequency is sweeped at 1 -4 MHz with sweep rate ranging from 10 mS to 100 mS. The solid state RF amplifier with output power of 80 watt is built. In phase power splitter is developed to feed the power to eight ion clearing electrodes. RF voltage at each port is then boosted to 200 volt by transmission line type high voltage hybrid transformer and 1 KV DC voltage is superimposed with RF at these transformers. Low power (2 watts) module is designed with RF transistors 2N 5070 operating in class AB push pull mode and power module is designed with RF transistors S 50 -28 operating in class AB push pull mode. This RF amplifier along with SCR controlled 1 KV / 200 mA regulated DC power supply is housed in a 19" rack.  Thruline type reflectometer cum VSWR Protection System is used to monitor forward and reflected powers of RF amplifier and to switch off DC supply of RF amplifier in case of excessive reflection from the electrodes.

4 MICROWAVE system for injector microtron

Microwave system for the 20 MeV injector-microtron, developed indigenously, consists of a 5MW S-Band klystron, 130 kV klystron pulse modulator and a WR-284 wave-guide transmission line. The microwave power to the microtron accelerating cavity is transmitted by means of the wave-guide line which consists of four-port circulator, dual directional coupler, wave-guide pressurizing unit, microwave window and dummy loads. The power from a stabilized signal generator at 2856 MHz is amplified up to 200 W by means of solid state amplifier and a solid state modulator driven klystron driver amplifier chain. The pulse modulator is made of a regulated high voltage 15kV DC supply, a charging choke, charging diode assembly, pulse forming network, high voltage thyratron and 1:10 pulse transformer whose bifilar wound secondary is connected to the device to supply the beam voltage to klystron in pulses of 3.6  sec duration at 1 Hz repetition rate. A fully automatic dry air pressurization system has been incorporated to replace the nitrogen cylinder based system. An all-solid state pulse modulator, giving 4kV, 10 sec pulses, has been developed to drive the driver klystron of the microwave system.

All the technologies related to the microwave system like the development of pulse forming networks, pulse transformers, complete interlock systems, thyratron trigger drives, high voltage capacitive dividers and pulse current transformers, klystron high voltage deck, driver amplifier, waveguide sections, dual directional couplers, microwave windows and high power water loads are developed indigenously.  The microwave system has completed over 10 years (more than 44,000- hours) of continuous and successful operation.

Fig. 8.  5MW klystron and compact waveguide line with the Injector Microtron. On the back side is seen the klystron modulator.

Fig. 9 From top oscilloscope traces of  reflected power from cavity,  microtron output beam current 23 mA, microwave forward power detected pulse at 2.6 MW,  and electron emission current from cathode 560mA. Horizontal scale is 500nS/Div.